Early on in the film, when Reverend William Thorne (Robert Donat) and his wife in the vicarage, they are discussing a book being returned to them. It's a copy of The 39 Steps (by John Buchan). Robert Donat (Rev Thorne) played Hannay in Alfred Hitchcock's 1935 film of The 39 Steps.
Although the cathedral scenes were filmed in Lincoln Cathedral the West Window is a matte painting.
In William Thorne's sermon in Gilchester Cathedral, he relates a story about Cromwell's troops smashing the West Window in the English Civil War, and the difficulties faced by the workmen repairing it. This actually happened to the Great West Window of Winchester Cathedral.
The fictional village name Hinton St. John, is probably derived from the village Hinton St. George in Somerset.
Original literary work: "The West Window", short story by Frank Baker.